The amperometric electrochemical analytical technique, in its classical aspects, concerns the isolation and measurement of faradaic electron transfer currents of a chemical species undergoing electro-oxidation or electro-reduction in a dilute solution of the species. The technique utilizes the principle of masking of the electrical force field where the species carries a charge. This is typically accomplished by introducing a supporting electrolyte into the solution, which because of its dominant presence, effectively eliminates migration of the species due to the electrical force field attraction. Under ideal conditions, therefore, and after the layer of species adjacent to the electrode is depleted, a stabilized current flow is established which is limited by the rate at which the species diffuses into the depleted zone. Thus, the diffusion rate and hence the current is a function of the concentration gradient driving force, and its value thus may be used to deduce species concentration. The case is more complex for a flowing stream, but under the conditions of laminar flow, a mass transport diffusion rate is achievable that correlates well with species actual concentration. Further, since chemical species oxidize or reduce at characteristic potentials, the principle may also be applied to derive qualitative species identification information, using curve interpretation, for example, with voltametric scanning.
Since solvent systems begin to respond at other than low applied potentials, and thus produce non-species specific interferences, the working potentials generally are not large for any system. The most sophisticated electrochemical detector systems, for example, work approximately in the -2.5 volt to 0 volt range for the electro-reducible mode (cathodic polarization range), and from about 0 to +1.5 volt, for the electro-oxidizable mode (anodic polarization range).
Adequate sensitivity for trace analysis is also critically dependent on electrode selection. In this respect, relatively few electrode matrials are known to have satisfactory utility. Moreover, while it has been mentioned that thin layer phenomena under laminar flow conditions produces a stable boundary layer, making the electrochemical detection technique potentially applicable to the monitoring of flowing sample streams (as in the case when used with a chromatographic column), this further sophistication has been of generally narrow practical application because of electrode design and material problems.
In respect to specific systems, the classical dropping mercury electrode (DME) is with but a few exceptions limited to the electro-reducible mode of analysis (-2.5 to 0 volt), and is hence usually unsatisfactory for a large class of organic compounds that are electro-oxidizable, but not electro-reducible. There are rare exceptions of compounds that oxidize in the cathodic potential range, but these compounds are so infrequent as to permit the above generalization. Also, while a few literature publications have indicated limited success in adapting the DME electrode to continuous flow analysis, the conversion is generally considered unsatisfactory and little, if at all, used in the practical sense especially for trace analysis. Thus, the DME electrode is almost entirely confined to use with non-stirred solutions.
Noble metal electrodes have sometimes been considered for use in electrochemical analysis but are extremely prone to the formation of metal oxide films with consequent surface fouling problems. Also, there is little likelihood that the narrow potential working range of the noble metals would be considered entirely satisfactory even if surface fouling problems could be controlled acceptably.
Carbon-based electrodes (usually inferring spectroscopic grade graphite) are more inherently suited for electro-oxidation analysis and may also be used for electro-reductions over a range of about .+-.1.5 volts. Thus, carbon electrodes would be of particular interest for electro-active species like phenolics, including halogenated phenols, which are of great concern in the environment because of their toxicity to aquatic life. However, since the early demonstration of the feasibility of electrochemical detection for methyl substituted phenols, very little further work has been reported on the oxidation of simple phenols. Reasons for this are believed found in the electrode noise and filming problems encountered when operating carbon electrodes at the higher applied potentials required to oxidize phenols compared to aromatic amines. The problems with high residual currents and detector noise at carbon anodes, for example, are well documented but the techniques for reducing the noise by impregnating the electrodes with mineral oils, wax or similar organic-soluble substances are not considered adequate for non-aqueous solvent systems. For similar reasons, the popular carbon paste anode (graphite powder in a paste medium) is generally considered unsuitable for monitoring the aqueous-organic mixtures normally required to elute the more water-insoluble compounds from liquid chormatographic columns.
The prior art has also considered the feasibility of combining graphite powders with more solvent tolerant materials. In this respect, polyethylene, Teflon.RTM. and silicone rubbers have been specifically suggested. Such electrode forms have made little progress, however, in terms of acceptance by the art, and lack of optimum sensitivity would seem indicated. Thus reproduction of two such prior art structures using a polyethylene and also a Teflon matrix form, mixed with graphite powder, produced generally inferior signal definition characteristics, and in one case inifinite cell resistance and thus unsuitability of purpose.